In commercially available blood oxygenating systems employing membrane type fluid flow transfer devices, venous blood is directed to a collapsible reservoir, and from there it is passed through a separate heater and through a separate membrane device in which blood flows past one surface of a membrane, oxygen flows past the other membrane surface, and oxygen passes through the membrane and diffuses into the blood as dissolved oxygen.
If the reservoir or reservoir and heater are placed lower than the membrane device (e.g., on the floor during nonuse), blood will siphon from the membrane device, and gas bubbles and pockets will form in the blood owing to the lower pressure on the blood side of the membrane. These bubbles and pockets must be removed prior to use to avoid introducing them into the bloodstream.